World’s first affordable diesel hybrid powertrain

December 14, 2006 Zytek has already proven its knowledge of powertrains and electric vehicle many times over, on and off the racetrack. Now, in collaboration with a UK Government initiative, the British engineering consultancy has developed a novel diesel hybrid powertrain that will be affordable in the most popular market segments. The new technology will allow vehicle owners to drive across London’s extended congestion charge zone for just four pence. The low-cost, high-efficiency hybrid-electric drivetrain offers a realistic alternative to expensive proprietary systems and can be quickly implemented within the packaging constraints of compact European passenger cars. A demonstration vehicle, built within the UK Government’s Ultra Low Carbon Car Challenge (ULCCC), has exceeded all targets set for the programme, delivering an exceptional 85g of CO2 per km compared with 121g/km for the already exceptionally efficient standard vehicle.

“Hybrid technology is now well established as a route to low emissions and outstanding fuel consumption, but so far it has carried a substantial price premium,” explains Zytek sales and marketing director Steve Tremble. “We wanted to develop a drivetrain that would make the many benefits of hybrid drive affordable within the most popular class of vehicle. We are delighted that the ULCCC, managed by the Energy Saving Trust, has supported this programme and allowed us to demonstrate the viability of our technology.”

Introducing the new technology at an event in London on December 5, Minister of State for Transport Dr Stephen Ladyman said: "If we are going to cut carbon emissions from road transport we need innovative new technologies. This launch shows that there is the potential to dramatically reduce emissions from cars, which is why the UK Government provides £5 million a year in a grant funding programme for investment in innovative low carbon road transport technologies."

The Ultra-Low Carbon Car Challenge is a two year, Government-supported programme to develop an affordable, five door vehicle with well-to-wheel CO2 emissions of less than 100g/km. To be eligible, the resulting vehicle must retain the space, features and safety of a conventional car in this class, whilst simultaneously delivering adequate performance. It must also be capable of reaching production within four to eight years at a commercially feasible price.

“The easy solution is to develop a premium vehicle that will be affordable when produced in volumes of hundreds of thousands a year,” says Tremble. “We wanted to develop something that breaks the mould, which can be accessible to many more people and therefore have a substantial impact on total road-car carbon emissions. To achieve this we took a fresh look at the available technologies and came up with a new, elegant yet very simple engineering solution.”

Another issue with today’s production hybrid drivetrains is that most rely on licensing agreements with the global leader in hybrid vehicles. “We wanted to develop a true alternative that gives vehicle manufacturers new opportunities in this fast-growing market,” says Tremble.

A novel dual-mode solution

Today’s passenger car production hybrids are either parallel or dual mode systems. Pure series hybrids offer high efficiency at low speeds but this greatly reduces as speed increases, and hence this drivetrain only finds application in low speed, stop start vehicles such as inner city buses. Parallel hybrids have the internal combustion engine permanently coupled to the wheels so are more efficient at high speeds, but offer less flexibility to optimise the drive for low speed duty cycles.

A new generation of dual mode transmissions offer the ability to operate in series or parallel mode so the drive can be optimised for each operating condition, but they are generally more complex and therefore larger and more expensive. There are also inevitable compromises in today’s production dual-mode designs.

Zytek’s novel solution is a dual-mode hybrid that eliminates these compromises by using sophisticated control strategies to replace clutches and complex epicyclic gearsets. In addition to the many advantages of dual-mode operation, this reduces component count, eliminates wearing friction surfaces, reduces packaging volume and substantially reduces manufacturing costs. The design also allows each motor to be optimised for its function within the drivetrain.

An affordable diesel hybrid transmission

To take the substantial step in efficiency needed to exceed the ULCCC emissions target, Zytek decided that its new hybrid should be built around a small, very high efficiency diesel engine. With common rail injection systems and sophisticated aftertreatment, the latest diesels provide quiet operation and exceptional fuel economy, combined with very low tailpipe emissions. They also work well in hybrid transmissions because the electric drive can be used to disguise turbo lag and give exceptional driveability.

The company already had a long heritage of electric and hybrid vehicle engineering programmes with DaimlerChrysler’s smart brand, so the 1.5 litre, three cylinder smart forfour CDi was the natural choice of base vehicle. DaimlerChrysler was extremely supportive and provided a vehicle and technical data for the project.

The biggest challenges for Zytek were packaging and cost. The help solve these, programme manager Neil Cheeseman decided to develop a bespoke hybrid transmission that would directly replace the existing gearbox without any changes to the vehicle body or powertrain mounting points. This novel dual-mode technology would also completely eliminate the usual licensing issues associated with the use of hybrid vehicle epicyclic gearboxes. To keep cost and engineering effort down, the new transmission would use largely proven production components.

The dual mode architecture has two motors: a starter/generator permanently engaged with the engine and a traction motor. Both are specifically designed for their application and engaged with the drivetrain when required via a dog synchromesh system similar to that found in automotive manual transmissions. A highly sophisticated control system, a key Zytek strength, takes the place of the synchro cone by automatically matching the speed of the input shaft and the lay shaft to allow seamless shifts with zero torque interruption. The technology uses experience from Zytek’s highly successful A1 Grand Prix engine and gearbox, which last season completed more than one million clutchless gearchanges at race speed, without a single failure.

The starter/generator is a 43kW DC brushless permanent magnet device capable of operating at speeds up to 3,600rpm. It is integrated into the gearbox bellhousing and sized to allow engine cranking at temperatures down to -40oC. It also provides torque assist when required to boost acceleration. Supplementing drive torque in this way allows a single very high gear to be engaged at low revs (1,600rpm) to keep the engine operating in its most efficient range. Cooling is provided by means of a water jacket integrated within the diesel engine’s cooling system.

The traction motor is a 50kW AC brushless permanent magnet device mounted on the end of the gearbox and capable of operating at speeds up to 11,500rpm. Cooling is provided by a dedicated system shared with the inverter. Testing of the traction motor has shown a peak efficiency of more than 97 percent. Although it shares the same fundamental architecture as the starter/generator, Zytek has chosen a different internal construction to optimise the device for low speed, high torque operation.

Connecting these devices with the final drive is a new, highly innovative gearbox developed by Xtrac to Zytek’s specification. Power inputs are received from the engine and starter/generator on one shaft and from the traction motor on a second shaft. These are blended under electronic control and distributed to the front wheels via the existing driveshafts.

When the vehicle pulls away, torque is delivered only from the electric traction motor. As speed increases, electric and diesel drive blend seamlessly until torque from the electric drive is reduced to zero. A very high input drive ratio allows the diesel engine to operate within the most efficient part of its power curve, with a low engine speed allowing efficient combustion and keeping frictional losses to a minimum. The broad power range of the dual mode system, combined with the phasing of electric and diesel drive at the ends of the vehicle speed range, allows the gearbox to operate with just two ratios - one for diesel drive, one for electric drive - without compromising NVH or economy.

“Achieving the target length, with the required inputs and outputs, was a real challenge,” says Robin Price, Business Development Manager with Xtrac. “We met Zytek’s specification through some very sophisticated CAE analysis and use of our own X36 gearsteel to allow smaller, lighter components.” The firm’s experience designing compact, high-power gearboxes such as that in the 2006 Le Mans winning Audi ensures that the solution will also be highly durable.

The new gearbox is just 138mm long, allowing the highly integrated hybrid transmission, including the gearbox and both electric motors, to package in the same space as the six speed Getrag automated manual transmission it replaces.

Powerful vehicle control

Key to obtaining the optimum efficiency from a hybrid powertrain is the interaction between the engine and hybrid control systems, and it was for this reason that Zytek elected to develop a brand new diesel control system. Taking the place of the existing unit on the car, the Zytek DEMS (Diesel Engine Management System) interfaces with every sensor and actuator on the standard engine and calculates the optimum fuel rail pressure, EGR (Exhaust Gas Recirculation) rates, turbo boost pressure, swirl valve position, and ultimately fuel injection quantity, timing and number of injections. The control system runs up to 5 injections per combustion event, depending on temperature, engine speed & load, and NVH requirements.

The Zytek DEMS also communicates with other systems via the standard vehicle’s LIN and CAN databusses, with a further CAN bus added to interface with the additional componentry. Building on the company’s history of developing petrol engine control systems, DEMS was developed and calibrated by Zytek in less than 14 months. The calibration enables the vehicle to exceed Euro IV emissions regulations without any other modifications to the engine beyond the fitment of a particulate trap.

Integrated into the DEMS control unit is a separate control board to manage the hybrid systems on the vehicle. In this configuration, the unit is referred to as the EHCM (Engine & Hybrid Control Module). To calculate the optimum blend of diesel and electric torque, the EHCM compares the throttle position, representing the driver’s demand for power, with factors such as battery state of charge, vehicle speed and safety inputs (required to validate the torque request, part of the safety system fitted to the vehicle). When decelerating, the EHCM calculates the optimum blend of friction and regenerative braking based on the position of the brake pedal. When only low rates of retardation are required, only regenerative braking is used. For higher rates of retardation, regenerative braking is blended with conventional friction braking using the vehicle’s standard braking system.

Battery and electrical systems

The battery, mounted on the boot floor flush with the loading lip, is a lithium ion unit supplied by Gaia. Compared with today’s Nickel metal hydride (NiMH) batteries, this chemistry offers very high energy density at a potentially lower production cost. “We surveyed all available technologies, including NiMH cells and electrochemical supercapacitors, and concluded that the Gaia Li-Ion solution offers the best combination of weight, cost and packaging volume for hybrid vehicles,” says Neil Cheeseman, project manager for the ULCCC hybrid smart forfour. “Our research has shown it has considerable potential for production applications.”

Battery cells are cooled with cabin air blown through the battery compartment, exiting through new vents in the rear of the vehicle. As the vehicle is a plug-in hybrid, an on-board charger has been fitted that can recharge the hybrid battery in under seven hours from 0-100% SOC (state of charge). The charging system is mounted in the spare wheel well under the battery and should not need servicing. The other key power component, the inverter, is mounted under the bonnet, close to the electric motors to minimise electrical losses. It’s innovative, software-managed current control system is used for both DC and AC machines, providing vehicle manufacturers with substantial design flexibility and the possibility of component sharing across different architectures.

As well as supplying the 300V three phase power for the motors, Zytek’s inverter supplies 1,800W of 12V power for the vehicle’s electrical systems.

The battery can be charged in three ways: via regenerative braking to efficiently capture the vehicle’s kinetic energy instead of dissipating it as heat; via the engine driving the starter/alternator to generate electricity; and by plugging the vehicle in to a conventional domestic 13Amp electrical socket using a connection hidden under the rear numberplate. Two additional safety pins on the plug, which is carried with the vehicle, disable the powertrain while it is connected to the mains supply.

Further fuel consumption savings have been achieved by replacing parasitic systems with more efficient and controllable electrically driven systems. When the engine is running, the flow of coolant is much more closely controller via use of an electrically-driven water pump, only consuming power when necessary. The brake vacuum pump has been supplemented with an electric pump so braking assistance can be provided even when the engine is off.

There is also a 6kW electric water heater to provide cabin warmth when the vehicle is operating as an electric vehicle, and a 5kW electric air conditioning compressor to provide all the cabin cooling requirements.

The electric water heating element is also used to dissipate energy when the batteries are full and the vehicle is slowing using regenerative braking. This helps the engine and cabin heating system warm up more quickly and provides a more consistent brake feel. “An important objective for this vehicle is that it is totally ready for production,” says Cheeseman. “It has the design detail and sophistication needed to enter the market immediately.”

Driving the Zytek dual-mode hybrid

The driver of the Zytek dual-mode diesel hybrid can choose from standard or high regenerative braking and can also select one of three control calibrations:

• ‘Economy’ is ideal for city driving. Use of the engine is minimised and maximum power is limited to increase the range on pure electric power.

• ‘Drive’ is ideal for mixed usage. Full power is available and the engine will automatically stop/start when appropriate.

• ‘Sport’ disables the stop/start function and allows maximum power and acceleration

The high regen mode is available in all of the above calibrations, allowing the driver to store more regenerative energy, giving even greater fuel economy and lower emissions.

The sophisticated control system makes powerblending imperceptible to the driver, with acceleration from zero to maximum with no torque interruptions. Acceleration from zero to 62mph (100kph) takes just 12 seconds and the car can cruise comfortably at highway speeds. With perfectly balanced corner weights, the vehicle’s dynamics are also comparable with the standard car. In all ways, the Zytek dual-mode diesel hybrid offers the driving experience of a conventional small family car.

Summary

Zytek’s contribution to the Ultra Low Carbon Car Challenge has created a new hybrid powertrain technology that for the first time makes ultra-low carbon emissions affordable in the most popular classes of vehicle. Using zero emissions electric drive and energy from an overnight recharge, the vehicle can make a return journey across London’s extended congestion charge zone for just eight pence. The innovative dual-mode diesel hybrid comfortably exceeds all the targets set by the ULCCC, with better performance and lower emissions, and is ready for applications engineering to bring it to production.

“For the first time, we have a simple, affordable hybrid solution. And it’s British,” says Stephen Hart, transport programme manager with the EST. “This is a considerable achievement. There is currently no production diesel hybrid and nothing that meets comparable cost targets. With Zytek’s technology, I can see this changing fairly soon.”
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